NEW YORK (Reuters Health) - Hospital and nursing-home employees who are infected with latent tuberculosis may often decline drug therapy to prevent the disease from becoming active, a new study suggests.

The study, which followed patients with latent TB at 32 U.S. and Canadian medical clinics, found that 22 of 53 healthcare workers who were offered treatment declined to take it.

Of all 720 patients in the study, 17 percent refused drug therapy to ward off active TB.

Compared with other study patients, healthcare workers -- all employees of a hospital or nursing home -- were nearly five times as likely to decline treatment, according to findings published in the medical journal Chest.

Latent TB refers to a chronic, but symptomless and non-contagious, infection with TB bacteria; it is detected through tuberculin skin testing. Such TB infections are common throughout the world because in most people, the immune system is able to suppress TB bacteria. Up to 15 million Americans are estimated to have latent TB infection.

The danger with latent TB is that it can become active; active TB usually attacks the lungs, causing symptoms such as a severe cough, chest pain and weight loss. It can also spread through the air from person to person.

The U.S. Centers for Disease Control and Prevention (CDC) recommends treatment of latent TB in people who are at high risk of progressing to active infection, including people whose immune systems are compromised -- due to HIV or immune-suppressing medications, for instance.

Treatment is also recommended for people who could easily spread the disease if it becomes active -- such as prison inmates and patients and employees at hospitals and nursing homes.

It's not clear why healthcare workers in this study were more likely than other people to decline drug treatment. One possibility is that they felt their risk of developing active TB was not significant, according to lead researcher Dr. C. Robert Horsburgh Jr., of the Boston University School of Public Health.

The average risk of developing full-blown TB disease is roughly one in 20, Horsburgh told Reuters Health in an email.

"It may also be that they are more concerned about side effects of the medicines used to treat latent TB," he said, "since they see side effects of medications every day in the patients they take care of."

Treatment of latent TB lasts for months. The standard therapy is a drug called isoniazid, usually prescribed as a nine-month course. Another option is a four-month regimen of a medication called rifampin.

The more common side effects of isoniazid include stomach upset and diarrhea; the drug has also been linked to cases of sometimes severe liver damage. Rifampin may cause symptoms such as stomach upset, nausea and headache.

In their study, Horsburgh and his colleagues found that healthcare workers, besides being more likely to decline treatment, were at increased risk of failing to complete treatment. Of the 209 prescribed treatment, 125 did not finish the full course.

Residents of congregate settings -- nursing homes, shelters and prisons -- were also at increased risk of not completing drug treatment.

One factor that did boost treatment compliance overall, however, was shorter therapy.

Most study patients who accepted treatment were placed on nine months of isoniazid, but those given four months of rifampin were more likely to complete treatment -- about 65 percent did, versus roughly 45 percent of those on isoniazid.

The implication, according to Horsburgh's team, is that shorter courses of therapy would encourage more patients to comply. They point out, though, that the effectiveness of the four-month rifampin therapy has not yet been tested in large clinical trials.

An ongoing clinical trial is testing the effectiveness of a three-month course of isoniazid plus another drug, rifapentine.

"I hope that new regimens of three months...or less will be shown to be effective and will replace the current regimens," Horsburgh said.

The study was funded by the CDC; one of the co-researchers has received grants from Pfizer, which makes the TB drug Mycobutin (rifabutin), and Oxford/Immunotec, which makes a blood test for TB.